COHERENT

Coherent is an adjective meaning logically connected; in writing, ideas are consistent and well organized. The term derives from the Latin cohaerens, meaning to stick together. It is also used more broadly to describe systems with unified, compatible components.

In physics, coherence refers to the correlation between physical quantities in a wave so that there is a fixed phase relationship. Temporal coherence describes the stability of phase over time, while spatial coherence describes correlation across different points in space. Coherence length and coherence time measure how long a wave maintains a predictable phase relationship. Coherent sources, such as lasers, produce light with a well-defined phase and can produce high-contrast interference, whereas incoherent sources produce light with random phase. Coherence is essential for interference and holography.

In quantum mechanics, coherent states are specific quantum states of systems such as the harmonic oscillator that resemble classical motion and minimize the uncertainty between conjugate variables. In quantum optics, coherent states underpin many descriptions of light and enable certain kinds of quantum information processing.

In communications and signal processing, coherent detection or coherent demodulation uses knowledge of the carrier’s phase to extract the signal, enabling higher sensitivity and spectral efficiency compared with non-coherent methods; it requires phase synchronization.

In linguistics and cognitive science, coherence refers to the overall logical and semantic connectedness of a text or discourse, achieved through coherence relations and cohesive devices that tie sentences and ideas together.